Copper sulphate (CuSO.sub.4.5H.sub.2 O) is a compound used extensively in such areas as agriculture (as a soil additive in pesticides and as a feed additive), medicine, electric batteries, steel manufacture and the petroleum industry.
The difficulty in providing a starting point for manufacture of copper chemicals has always been the relative inactivity of the metal. The metal can be oxidised by heating in air, but this is expensive and slow, and the resultant oxide is not satisfactory as a source of cupric salts.
Traditionally, copper sulphate has been manufactured by reacting copper metal, usually copper scraps, with dilute (25%) sulphuric acid, in the presence of excess air, utilising the reaction: EQU Cu+H.sub.2 SO.sub.4 +1/2O.sub.2 .fwdarw.CuSO.sub.4 +H.sub.2 O (1)
The reaction is endothermic and will not proceed at a satisfactory rate unless the temperature of the solution is maintained at a minimum of 70.degree. C., and usually at more than 80.degree., for example to a temperature in the range 80.degree. C. to 90.degree. C.
An external heat source is required for this reaction, and steam is commonly used for this purpose.
The reaction is usually carried out by pumping the acid through towers packed with scrap copper, through which air is blown, or in submerged reactors, where the acid is agitated in a tank packed with scrap, and air is introduced by submerged aerators.
Air addition rates are usually required to be 7 to 10 times the stoichiometric amount, based on its oxygen content, giving an oxygen level of 7 times the stoichiometric amount.
The reaction is usually conducted batchwise, and is complete when the residual acid level falls to an acceptable level.
With such a process, production rates of 0.1 t/hour/m.sup.3 of reactor volume are rarely exceeded.
The resultant copper sulphate solution is cooled and the copper sulphate crystallises out and subsequently removed, usually by centrifuge. The crystals are dried and packed and the mother liquid is usually returned after acid enrichment, to the next batch.
This traditional method has several problems:
(a) Capital and maintenance costs must take into account the highly corrosive nature of aerated 25% hot sulphuric acid.
(b) The endothermic nature of the reaction, combined with the heat removal resulting from the high air flows required, result in the need for external heating either by steam or some other means.
(c) The process is non-selective in that metals other than copper in scrap, that will dissolve in hot sulphuric acid, can contaminate the product.
(d) The reaction rate is not fast, and production rates of 0.05 t/hour/m.sup.3 of reactor volume are common.
(e) The air effluent can contain sulphur dioxide, acid droplets and copper sulphate mist, and must be treated in some way.
(f) The necessity to recycle mother liquor leads to build-up of contaminants which must be removed by some method.
As a result of at least some of the above problems, energy costs may amount to in excess of $50/t of product.
It has also been long known that solutions of ammonia salts, or various combinations of them, will also attack copper metal, gain in the presence of oxygen. Copper ammines are produced, according to the equations: EQU Cu+1/2O.sub.2 +4NH.sub.3 +H.sub.2 O.fwdarw.Cu(NH.sub.3).sub.4 (OH).sub.2 (2)
and EQU Cu+1/2O.sub.2 +2NH.sub.3 +2NH.sub.4 X.fwdarw.Cu(NH.sub.3).sub.4 X.sub.2 +H.sub.2 O (3)
or EQU Cu+1/2O.sub.2 +2NH.sub.3 +(NH.sub.4).sub.2 Y.fwdarw.Cu(NH.sub.3).sub.4 Y H.sub.2 O (4)
in the case of dibasic acid salts.
U.S. Pat. No. 3,760,070 discloses a process in which a reaction according to equation 3 is carried out. The copper ammine solution is then heated to produce copper oxide, and the ammonia and ammonium salt are recycled.
One major disadvantage of the process of U.S. Pat. No. 3,760,070 is the energy costs associated with the heating step.
An alternative ammonia/ammonium salt process uses organic strippers.
In this process copper scrap is reacted with a solution containing ammonia and ammonium carbonate, with aeration, under agitation. This solution rapidly dissolves copper up to a concentration of 30 g/l. The pregnant solution is then removed from the reactor and mixed with a proprietary organic liquid (for example one known as LIX54) which although immiscible with the aqueous ammine solution, has the power of stripping the pregnant solution of the contained copper, while leaving the NH.sub.3 and Cu.sub.3.sup.= in the aqueous phase. The stripped aqueous phase is then recycled to the reactor, while the separated organic layer, containing the copper ions, is reacted with dilute sulphuric acid, which forms copper sulphate. This migrates to the aqueous phase, leaving the organic layer stripped of copper.
The organic layer is then separated for re-use, while the aqueous phase is evaporated, cooled, and the copper sulphate recovered in the usual way.
This method has the following advantages:
(a) Maintenance costs are lower because the reacting medium is alkaline.
(b) The reaction is exothermic, and is therefore self-sustaining, energy wise.
(c) The process is selective, as only those metals soluble in ammonia are attached.
(d) The reaction rate is quite fast and rates of 0.1-0.2 t/hr/m.sup.3 of reactor volume are common. but also has the following disadvantages:
(a) Capital costs for machinery are reasonable, but the cost of the organic stripper can be as high as $20,000 m.sup.3 of reactor volume.
(b) Losses of organic stripper can usually be reckoned to be in the order of $10 to 50 per tonne of copper sulphate product.
(c) The organic stripper is prone to what is known as "crudding up" due to impurities, leading to even higher losses.
(d) Ammonia recovery systems are required to recover expensive ammonia from the effluent gas and solid residues.
Generally all except very large producers have remained with the "lesser of the two evils" and have used the acid route.
Various organic strippers have also been developed for stripping copper ions out of acid copper sulphate solution, but these also suffer from the problem of reagent loss and "crudding" referred to above.
It is an object of this invention to provide an improved process for producing copper compounds.
The invention provides, a method of producing copper compounds, including the step of contacting metallic copper with oxygen or an oxygen-containing gas, and an aqueous solution consisting essentially of water in solution in which, to a concentration up to the limit of solubility thereof, is a soluble ammonium salt NH.sub.4 X, where X is the anion of said salt, together with ammonia in an amount such that initially said solution is alkaline, whereby as a result of said contact said metallic copper is initially dissolved to form a copper ammine Cu(NH.sub.3).sub.4 X said ammine formation proceeding until the saturation concentration of said ammine is reached, whereafter said ammine continuously breaks down to form the water insoluble tribasic salt 3Cu(OH).sub.2.CuX.sub.2, and the water-soluble products of the decomposition of said ammine continuously reform the said ammine by further reaction with said metallic copper and said oxygen or oxygen-containing gas until said solution becomes exhausted of the said anion.
The invention also provides apparatus for carrying out a method of producing copper compounds, including
a cylindrical reaction tank fitted with a circular screen such that copper metal is confined to the area between the screen and the tank walls, and having a bladed agitator located inside the screen driven at a minimum tip speed of 500 meters/minute, and also providing for the introduction of an oxygen containing gas at or near the agitator blades;
said tank being provided with means of adding acid at a controlled rate; and
said tank being fitted with a solids removal system such that the said water insoluble tribasic copper salt is removed from the reacting solution at a controlled rate.
It has been discovered that if an aqueous solution of an ammonium salt of any of the common acids (for example sulphuric, hydrochloric or nitric) is prepared at any strength up to saturation level, and this solution is made alkaline with aqueous ammonia, preferably to a pH of 7.0 to 8.0, and this solution is then allowed to react with copper metal in the presence of oxygen, then an aqueous medium is produced, which is capable of rapid conversion of copper metal, in the presence of oxygen, into the insoluble tribasic salt, under certain minimum conditions of agitation and aeration, until all the acid ions contained in the original aqueous solution are exhausted.
Furthermore, if the acid ions removed from the original solution, by formation of the insoluble tribasic salt, are replaced by addition of the relevant acid to the aqueous medium at an equivalent rate, after the formation of the tribasic salt has begun to occur, and if the resultant solid tribasic salt is removed from the aqueous medium by, for example, filtration, then a continuous cyclic reaction occurs where copper ammine is continuously formed from the copper metal, and is continuously decomposed to the tribasic salt, at a rapid rate.
The nett effect is a continuous process for conversion of copper metal into solid 3Cu(OH).sub.2.CuSO.sub.4 (Brochantite) in the case of ammonium sulphate, 3Cu(OH).sub.2.CuCl.sub.2 (Atacamite or copper oxychloride) in the case of ammonium chloride, or 3Cu(OH).sub.2.Cu(NO.sub.3).sub.2 in the case of ammonium nitrate.
These tribasic salts can easily be converted into the relevant acid salt by addition of a stoichiometric quantity of the relevant acid, according to the equation EQU 3Cu(OH).sub.2 CuX.sub.2 +6HX.fwdarw.4CuX.sub.2 +6H.sub.2 O (5)
or EQU 3Cu(OH).sub.2 CuY+3H.sub.2 Y.fwdarw.4CuY+6H.sub.2 O (6)
in the case of a dibasic acid.
This process can be controlled, if necessary, to produce direct crystalline salts, by use of the acid in concentrated form.
Equally, the tribasic salt can easily be converted to copper hydroxide by addition of a stoichiometric quantity of an alkali such as sodium hydroxide, according to the equation EQU 3Cu(OH).sub.2 CuX.sub.2 +2NaOH.fwdarw.4Cu(OH).sub.2 +2NaX (7)
and EQU 3Cu(OH).sub.2 CuY.sub.2 +2NaOH.fwdarw.4Cu(OH).sub.2 +2NaY (8)
the insoluble hydroxide can be recovered by filtration and washing.
However, it should be noted that the tribasic salts themselves, particularly the sulphate and chloride, find direct application as fungicides. The invention thus also provides a direct method for the manufacture of such tribasic salts.
The reaction mechanism appears to involve initial formation of copper ammine, according to the equation EQU Cu+1/2O.sub.2 +4NH.sub.4.sup.+ .fwdarw.Cu(NH.sub.3).sub.4.sup.++ +H.sub.2 O+2H.sup.+ ( 9)
This reaction proceeds until the copper content of the solution approaches 28 g/l. At this point the ammine decomposes according to the equation EQU 4Cu(NH.sub.3).sub.4.sup.++ +8X.sup.- +6H.sub.2 O.fwdarw.3Cu(OH).sub.2.CuX.sub.2 +10NH.sub.3 +6NH.sub.4.sup.+ +6X.sup.-( 10)
in the case of a monobasic acid salt, or EQU 4Cu(NH.sub.3).sub.4.sup.++ +4Y.sup.- +6H.sub.2 O.fwdarw.3Cu(OH).sub.2.CuY+10NH.sub.3 +6NH.sub.4.sup.+ +6Y.sup.-( 11)
in the case of a dibasic acid salt.
This results in the formation of the tribasic salt, accompanied by the release of all of the contained ammonia and most of the contained acid anions, which are then available for the formation of further copper ammine.
Thus the reaction continues to produce tribasic copper salt, while maintaining the copper ammine level in the reaction liquor at the saturation concentration.
The reaction can plainly be seen to only continue whilst sufficient acid anions are available for formation of the tribasic salt, because only 75% of the anion is released.